1. Field of the Invention
This invention relates to a method, an apparatus and a process. More specifically, this invention concerns a method for synchronization of the biasing potential between a plurality of development electrodes and a backing electrode with the successive travel of an electrostatically imaged discontinuous insulating sheet.
2. Description of the Prior Art
The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well-known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on the imaging surface of an imaging member by first uniformly electrostatically charging the surface of the imaging member in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light struck areas of the imaging layer are thus rendered conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on the this image bearing surface is rendered visible by development with finely divided colored electroscopic materials, known in the art as "toner". Depending upon the relative polarity of the charge of the toner and the latent electrostatic image, the toner can be attracted or repelled by said image. In the event that the toner is of an opposite polarity to the latent image, it will be attracted thereto and thus form a positive reproduction of the original. Alternatively, in the event that the toner and the latent image are of similar polarity, the toner will be repelled by the charge pattern and attracted to the nonimaged or background areas thus forming what is commonly referred to as a reversal image.
The developed image can then be read or permanently affixed to the photoconductor where the imaging layer is not to be reused. This latter practice is usually followed with respect to the binder-type photoconductive films (e.g. zinc oxide in a film forming insulating resin) where the photoconductive imaging layer is also an integral part of the finished copy.
The so-called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to an insulating member, such as a sheet of paper and thereafter developed. Where the latent image is transferred to an insulating member it can be rendered visible by development with either dry or liquid developers. In liquid development systems, (also hereinafter referred to as electrophoretic development systems) charged developer materials dispersed in an insulating liquid are brought in contact with the latent image by the establishment of a biasing potential across the liquid whereby the charged developer materials move under the influence of a unidirectional field toward the latent image bearing surface of the insulating sheet. Where the developer materials and the latent image are of different polarities, the developer is readily attracted and attached thereto. Alternatively, where the developer and the latent image are of the same polarity, the relative electrostatic forces result in repulsion of said particles. These particles thus attach only to those areas on the insulating sheet having little or no charge density. In this later situation, the retention of developer particles is not assisted by any forces present on the insulator but rather is principally dependant upon the magnitude of the impelling electrical force in the developer liquid for their adherence to this surface.
In reversal development, developer particles are generally attracted only to those areas where there is little, if any, charge density, thus, producing very poor continuous tone reversal images. In order to improve the continuous tone quality of such reversal images, the DC voltage applied to the development electrode should preferably be equal to the surface potential of the area of maximum charge density of the latent electrostatic image. This will provide an electric field between the development electrode and the insulating layer in proportion to the difference between the maximum charge density and the charge density of the less highly charged areas.
The above arrangement provides satisfactory development of reversal images having good continuous toner quality. In carrying out the above development process, it has been common practice to use a continuous insulating sheet. This provides a barrier to deposition of developer materials on the backing electrode during the maintenance of an electrical bias between the development and backing electrodes. In the event, that cut sheets (hereinafter referred to as discontinuous insulating members) are used, the electrostatically imaged insulating member would first have to be placed between the two electrodes and the apparatus thereafter activated. Upon completion of development of the latent image on the insulating member, the apparatus would then have to be inactivated and the developed insulator removed. This type of sequence is not practical where the only force maintaining the developer particles on the surface of the insulating member is the field created by the electrical bias between the two electrodes. Upon removal of said bias, at least some developer particles will diffuse from the surface of the insulating member thus partially destroying the visible image pattern thereon. In order to prevent this from occurring, the electrical bias must be maintained during and subsequent to removal of the insulating member from the development zone intermediate between these electrodes. Where such member is in the form of a discontinuous sheet, its gradual withdrawal from said zone under such conditions will permit the deposition of developer particles on the backing electrode in those areas where said insulator no longer separates the two electrodes. This deposition of developer particles on the backing electrode is not only wasteful of such developer materials but also interferes with the establishment of an electrical bias between said electrodes in the development of successive latent electrostatic images.
It is, therefore, the object of this invention to provide a method for the electrophoretic development of reversal images on discontinuous insulators devoid of the above noted deficiencies.
More specifically, it is the object of this invention to provide an electrophoretic development method wherein developer materials are selectively attracted to an electrostatically imaged discontinuous insulating member during its passage through and withdrawal from the developer liquid.
It is a further object of this invention to provide an apparatus suitable for use in the above method.